1. Field of the Invention
This invention relates to a pick-up tube and more particularly to a pick-up tube of the photoconductive target type that is light biased by a bifurcated light pipe.
2. Description of the Prior Art
Of the photoconductive image pick-up tubes, those using lead monoxide (PbO) as a main component of the photoconductive layer and those having their photoconductive layer composed mainly of selenium, arsenic and tellurium (Se, As, Te) have excellent image pick up qualities and are used often in broadcast cameras. However, these tubes also have a certain length of delay in rise or fall time, which delay adversely affects the reproduction of the original picture, particularly at low light levels. It should be here noted that the delay in the rise time, or build-up lag, is the interval of time from the instant that the light from the object falls upon the photoconductive layer to the instant that the photoelectric signal owing to the incident light is generated. The delay in the fall time, or decay lag, is the duration from the moment the incident light is interrupted to the moment the photoelectric signal due to the incident light vanishes.
In order to reduce the build-up and decay lags, it is known to increase the dark current level of the photoelectric signal by auxiliary illumination to add uniform light to the light from the object. This auxiliary illumination can be done by incorporating a weak light source in the tube or by utilizing the light from the filament of the tube cathode. By using a light conductor, this auxiliary light can be transferred, for example, to the collector space of the tube andfrom there distributed uniformly over the photoconductive layer by diffuse reflection.
As is known per se, the light conductor may consist of a glass rod which is suitably bent so that the light of a light source situated in front of one end of the rod emanates from the other end of the rod. Pick-up tubes incorporating such light conductors are shown in U.S. Pat. No. 3,628,076 to Weijland et al, issued on Dec. 17, 1971; U.S. Pat. No. 3,751,703 to Weijland et al, issued on Aug. 7, 1973; and U.S. Pat. No. 3,978,365 to Limper, issued on Aug. 31, 1976, which shows a bifurcated-type light conductor.
The light pipe must transmit enough light to be useful but should not transmit too much light so as to cause undue shading and other detectable uneven spurious signals. Often, variations in the photoconductive materials from tube to tube will necessitate different light intensities to obtain a desirable bias dark current. Also, the light transmission properties of the light pipes themselves may vary somewhat from pipe to pipe. Therefore, commercially available or standard "off the shelf" light pipes cannot often provide the light intensity that is required in view of these variations. Thus, it is desirable to have a light pipe that will account for such variations and produce a predetermined light transmission so that the required level of illumination and, hence, dark current may be consistently achieved.